1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, and more particularly, to a zoom lens suited for use in a broadcasting television camera, a cinema camera, a video camera, a digital still camera, and a silver-halide film camera.
2. Description of the Related Art
In recent years, a zoom lens having reduced size and weight, a wide angle of field, a high zoom ratio, and high optical performance is desired for use in an image pickup apparatus, such as a television camera, a cinema camera, a film camera, or a video camera. In particular, an image pickup device such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) used in a television or cinema camera as a professional moving image pickup system has a substantially uniform resolution over the entire image pickup range. Therefore, a zoom lens using the image pickup device is required to have a substantially uniform resolution from the center to the periphery of the screen. There are also needs for reductions in size and weight for an image pickup mode that places emphasis on mobility and operability.
As a zoom lens having a wide angle of field and a high zoom ratio, there has been known a positive-lead-type four-unit zoom lens in which a lens unit having a positive refractive power is arranged closest to the object and which consists of four lens units as a whole.
For example, Japanese Patent Application Laid-Open No. H06-242378 discloses a four-unit zoom lens having an F-number of about 1.6 to 1.7 at the wide angle end, an angle of field of about 65° to 87° at the wide angle end, an angle of field of about 4° to 13° at the telephoto end, and a magnification-varying ratio of about 8 to 18. The four-unit zoom lens is constituted of a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive or negative refractive power, and a fourth lens unit having a positive refractive power. Further, the first lens unit is constituted of a first sub-lens unit having a negative refractive power, a second sub-lens unit having a positive refractive power, and a third sub-lens unit having a positive refractive power, and is configured so that the second sub-lens unit moves to the image side for focusing from the infinity side to the proximity side.
Japanese Patent Application Laid-Open No. 552-084754 discloses a four-unit zoom lens constituted of a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. The first lens unit is constituted of a first sub-lens unit having a negative refractive power, a second sub-lens unit having a positive refractive power, and a third sub-lens unit having a positive refractive power, and is configured so that the second sub-lens unit moves to the image side for focusing from the infinity side to the proximity side.
Japanese Patent Application Laid-Open No. 2008-216480 discloses a four-unit zoom lens having an F-number of about 2.8 at the wide angle end, an angle of field of about 35° at the wide angle end, an angle of field of about 12° at the telephoto end, and a magnification-varying ratio of about 2.7. The first lens unit is constituted of a first sub-lens unit having a negative refractive power and a second sub-lens unit having a positive refractive power, and is configured so that the second sub-lens unit moves to the object side when focusing from the infinity side to the proximity side.
However, with the zoom lens disclosed in Japanese Patent Application Laid-Open No. H06-242378, realization of both the further reductions in size and weight and the high optical performance is difficult. Japanese Patent Application Laid-Open No. H06-242378 discloses in the embodiments 1 and 2 a configuration in which lens units each having a negative refractive power are arranged on the object side with respect to the stop. The diameters of the second lens unit and the lens units on the image side of the first lens unit tend to increase, which is disadvantageous in reducing the size of the zoom lens. When the refractive power of each lens unit is increased in order to reduce the size and weight, an aberration variation is increased over the entire zoom range, which makes it difficult to achieve high optical performance. Further, Japanese Patent Application Laid-Open No. 6-242378 discloses the embodiment 3 in which the third lens unit has a positive refractive power, but the high refractive power of the third lens unit increases the number of constituent lenses, which is disadvantageous in reducing the size and weight. In addition, in order to ensure enough F-number and exit pupil for the strongly convergent ray that exits the third lens unit, the number of constituent lenses of the fourth lens unit is increased, with the result that the reductions in size and weight are difficult.
On the other hand, Japanese Patent Application Laid-Open No. S52-084754 discloses an example in which the third lens unit has a positive refractive power, but the third lens unit has weak refractive power, with the result that the ray that exits the third lens unit diverges. In order to ensure enough F-number and exit pupil for the divergent ray, the number of constituent lenses of the fourth lens unit is increased, with the result that the reductions in size and weight are difficult.
With regard to the zoom lens disclosed in Japanese Patent Application Laid-Open No. 2008-216480, the refractive power and lens configuration of each lens unit is disadvantageous in further increasing the angle of field, which makes it difficult to suppress an increase in lens diameter accompanying the increase in angle of field.